Role of Positively Charged Amino Acids in the M2D Transmembrane Helix of Ktr/Trk/HKT Type Cation Transporters

Studies suggest that Ktr/Trk/HKT type transporters have evolved from multiple gene fusions of simple K+ channels of the KcsA type into proteins that span the membrane at least eight times. Several positively charged residues are present in the eighth transmembrane segment, M2D, in the transporters but not K+ channels. Some models of ion transporters require a barrier to prevent free diffusion of ions down their electrochemical gradient, and it is possible that the positively charged residues within the transporter pore may prevent transporters from being channels. Here we studied the functional role of these positive residues in three Ktr/Trk/HKT type transporters (Synechocystis KtrB mediated K+ uniporter, Arabidopsis AtHKT1 mediated Na